Suggested reading

Watch as a developer fixes a bug in a MediaWiki extension, including investigation, git commit, getting it reviewed and merged, and closing the Bugzilla ticket (now replaced by Phabricator).

Παρατηρήσεις, ερωτήσεις και υποστήριξη

You are expected to do some basic research yourself first: Look at the code, try to get some understanding what it is supposed to do, read related documentation, try to find the probable place(s) where you need to make changes in order to fix the bug.

If you have a specific question about the bug itself, comment in the corresponding bug report (normally a task in Phabricator). "Can you give me more info how to fix this bug?" is not a good question to start with: The more specific your questions are, the more likely somebody can answer them quickly. If you have no idea at all how to fix the bug, maybe that bug is not (yet) for you - please consider finding an easier one first.

When asking, explain what you have tried and found out already, so others can help at the right level. Be specific - for example, copy and paste your commands and their output (if not too long) instead of paraphrasing in your own words. This avoids misunderstandings.

Be patient when seeking input and comments. On IRC, don't ask to ask, just ask: most questions can be answered by other community members too if you ask on an IRC channel. If nobody answers, please ask on the bug report or wiki page related to the problem; don't just give up.

Communicate that you work on a task

Do not ask if you can work on a task.
You do not need to announce your plans before you start working on a bug, but it would be welcome.
You are welcome to set yourself as the assignee in a task: Use the dropdown Add Action… → Assign/Claim in Phabricator.
At the latest when you are close to proposing a patch for the task, it is good to announce in a comment that you are working on it.
Your announcement helps others to not work on the bug at the same time and to not duplicate work.

Also note that if a task already has a recent link to a patch in Gerrit and has the project "Patch-For-Review" associated in Phabricator, choose a different task to work on instead - avoid duplicating work.
If an existing patch in Gerrit has not been merged and has not seen any changes for a long time, you could also pick up that existing patch and improve it, based on the feedback in Gerrit and in the task.

If you stop working on a task, remove yourself as the assignee of the task, so others know that they can work on the task and don't expect you to still work on it.

By communicating early you will get more attention, feedback and help from community members.

Appendix

MediaWiki contributors at work in Bangalore, India.

PHP

MediaWiki is written in PHP, so you'll need to get familiar with PHP to hack MediaWiki's core.

Learn PHP

PHP tutorial — Available in many different languages. If you have no knowledge of PHP but know how to program in other object-oriented programming languages, PHP will be easy for you to learn.

MediaWiki currently uses MySQL and MariaDB as the primary database back-end. It also supports other DBMSes, such as PostgreSQL and SQLite. However, almost all developers use MySQL/MariaDB and don't test other DBs, which consequently break on a regular basis. You're therefore advised to use MySQL/MariaDB when testing patches, unless you're specifically trying to improve support for another DB. In the latter case, make sure you're careful not to break MySQL/MariaDB (or write queries that are horribly inefficient in it), since MySQL/MariaDB is what everybody else uses.

JavaScript and CSS

JavaScript and CSS have become omnipresent in front-end code.
You don't have to be familiar with JavaScript, jQuery and CSS to work on MediaWiki, but you might need to, depending on what you choose to work on.

↑MediaWiki runs on most platforms that can support PHP, however, the lack of certain utilities or operating system features may limit the functionality or performance of MediaWiki on non-LAMP platforms.